Clutch stack wear sensor system for a power transmission device

ABSTRACT

A power transmission device with a clutch plate wear sensor system includes a power transmission shaft configured to transmit torque and a clutch configured to selectively deliver power into or out of the power transmission shaft. The clutch includes a clutch stack and clutch piston that moves to selectively actuate the clutch stack between an engaged state to deliver power through the clutch and a disengaged state to not deliver power through the clutch. The clutch stack wear sensor system includes at least one detectable element arranged for movement in unison with the clutch piston and a sensor arranged to detect the detectable element and provide a signal indicative of a wear characteristic of the clutch stack.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Non-Provisional patentapplication Ser. No. 16/035,080, filed Jul. 13, 2018 (and issued as U.S.Pat. No. 10,914,327 on Feb. 9, 2021). The subject matter of thisapplication is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to transmission devices with clutch stacksand, more specifically, to a clutch stack wear sensor system that maymeasure both the rotational speed of a piston and the displacement ofthe piston due to wear of the clutch plates within a clutch stack.

2. Discussion of the Related Art

Traditionally, transmission drive shaft rotational speed can bedetermined with a sensor that determines the rotational speed of theshaft by relaying changes in the magnetic field caused by evenly spacedgrooves or ribs disposed on a ferrous component joined to the shaft. Acontroller can measure the distance between these changes in time andcalculate the rotational speed of the shaft based on the circumferenceof the component. During operation of the transmission, the frictionplates within a clutch stack wear and eventually wear to a point oflosing torque capacity.

SUMMARY OF THE INVENTION

A clutch wear sensor system is provided that may determine clutch wearbased on a signal that corresponds to a position of a detectableelement. The detectable element may be on a rotating clutch component,such as a piston that compresses a clutch stack during clutchengagement. The position of the detectable element is indicative of anaxial or longitudinal position of the piston within the clutch during anengaged state of the clutch, which corresponds to the amount of wear andtherefor thickness of the clutch stack. A sensor within the systemdetects the detectable element and may provide a signal that may beproportional to clutch wear for detecting clutch wear along a continuumor may provide a go/no-go indication or otherwise indicate one ofmultiple discrete clutch wear states. The detectable element may beimplemented as a defect(s) amongst other detectable elements to producean anomaly or irregularity within the signal that is recognized by thecontrol system as a wear indication, either as a replacement conditionor other worn-state indication that may correspond to an amount of wearfor predicting remaining clutch life. This may be done by configuringthe detectable elements and sensor to provide a signal(s) that isproportional to wear to facilitate predicting remaining clutch life.Instead of signals that are proportional to wear, the detectableelement(s) and sensor may be configured to provide a signal(s)indicative of discrete wear states. In one example, instead of adefect(s), the detectable element may be implemented to produce a signalthat itself corresponds to a discrete clutch wear state, which mayfacilitate providing a go/no-go indication or a replace-soon indication.Such non-defect implementations may include multiple markings or otherdetectable elements that may be provided in multiple stages or sets,with the different stages providing different signal characteristicsthat correspond to different wear states of the clutch. Anothernon-defect implementation may be implemented as a single marking orother detectable element that, when detected, indicates areplacement-needed condition, for example, as a go/no-go indication. Theclutch wear sensor system may be implemented along with a rotationalspeed sensor for both determining shaft rotational speed and alsoevaluating clutch wear by detecting the same set of detectable elements.Separate sensors may instead be used for determining shaft rotationalspeed and clutch wear by detecting different sets of detectableelements.

A sensor system is provided that includes a sensor that is configured tointeract with a rotating clutch piston to measure both the rotationalspeed of the piston and the displacement of the piston along thehorizontal axis resulting from wear of the clutch stack.

According to one aspect of the invention, as the friction plates of theclutch stack wear, the sensor detects further axial movement of therotating clutch piston for determining a wear state of the frictionplates of the clutch stack.

According to another aspect of the invention, a single sensor elementmay be arranged to detect both the rotational speed of the output shaftof the transmission and the wear of the friction plates within theclutch stack. By altering the outer surface of a component joined to theoutput shaft that also applies pressure to the clutch stack, a singlesensor may be used to detect both the rotational speed of the outputshaft and the wear of the friction plates within the clutch stack basedupon its detection of changes in the magnetic field caused by the changein the contour or composition of the outer surface of the componentduring operation of the transmission.

In accordance with an embodiment of the invention, a power transmissiondevice with a clutch stack wear sensor includes a piston coupled to apower transmission shaft and a clutch configured to selectively deliverpower into or out of the power transmission shaft. The clutch includes aclutch stack and a clutch piston that can move to selectively actuatethe clutch stack between an engaged state to deliver power through theclutch and a disengaged state to not deliver power to through theclutch. The clutch stack wear sensor system includes at least onedetectable element that moves in unison with the clutch piston and asensor arranged to detect the detectable element and provide a signalindicative of a wear characteristic of the clutch stack.

According to another aspect of the invention, the detectable elementincludes a plurality of detectable elements spaced around an outersurface of the clutch piston. At least one of the plurality ofdetectable elements may include a change in geometry. For example, atleast one of the plurality of detectable elements may include an angledportion that displaces the at least one detectable element from a firstlocation to a second location offset toward an adjacent detectableelement.

According to yet another aspect of the invention, the wearcharacteristic of the clutch stack may be associated with the pistonbeing in one of an operating position, an intermediate position, and areplacement position. When the sensor detects that the axial location ofthe piston is in the intermediate position, the sensor is able to trackmovement of the piston through the intermediate position. Further, thewear characteristic of the clutch stack may be tracked by the movementof the piston through the intermediate position. When the sensor detectsthat the piston is in the replacement position, a notification may beprovided to a user.

According to another aspect of the invention, the at least onedetectable element may include a first set of detectable elementsassociated with the operating position, the intermediate position, andthe replacement position of the piston, a second set of detectableelements associated with the intermediate position and the replacementposition of the piston, and a third set of detectable elementsassociated with the replacement position of the piston. When the sensordetects only the first set of detectable elements, it is determined thatthe piston is in the operating position. When the sensor detects thefirst and second sets of detectable elements, it is determined that thepiston is in the intermediate position. When the sensor detects thefirst, second, and third sets of detectable elements, it is determinedthat the piston is in the replacement position.

According to yet another aspect of the invention, the sensor may alsodetect a rotation speed of the piston by way of the plurality of textureelements. The rotation speed of the piston may be equal to a rotationspeed of the power transmission shaft.

In accordance with another embodiment of the invention, a powertransmission unit with a sensing system includes a piston coupled to apower transmission shaft, a plurality of detectable elements axiallyspaced around the outer surface of the piston, and a sensor disposedadjacent the outer surface of the piston to detect the detectableelements during rotation of the piston and power transmission shaft. Asfriction plates of a transmission clutch stack wear, the piston movesaxially along the transmission power shaft. The sensor detects an axiallocation of the piston by way of the detectable elements and provides asignal indicative of a wear characteristic of the friction plates, whilealso detecting a rotational speed of the piston by way of the detectableelements.

According to another aspect of the invention, at least one of thedetectable elements may include an angled portion oriented at an anglebetween 0° and 90°. Further, the axial location may be one of anoperating position, an intermediate position, and a replacementposition. When the sensor detects the axial location of the piston is inthe replacement position, a notification may be provided to a user.

According to yet another aspect of the invention, the at least onedetectable element may include a first set of detectable elementsextending a first axial length along the outer surface of the piston, asecond set of detectable elements extending a second axial length alongthe outer surface of the piston, and third set of detectable elementsextending a third axial length along the outer surface of the piston.The operating position of the piston includes the first detectableelements, the intermediate position of the piston includes the first andsecond detectable elements, and the replacement position of the pistonincludes the first, second, and third detectable elements.

In accordance with yet another embodiment of the invention, a powertransmission device may include a shaft for transmitting power out of orinto the power transmission device, a clutch stack including a pluralityof friction plates, a piston coupled to the output shaft and disposed ata second side of the clutch stack, and a sensor disposed adjacent theouter surface of the piston. The piston includes a first side adjacentthe clutch plate stack, a second side, and a plurality of detectableelements axially spaced around the outer surface of the piston. As thefriction plates of the clutch stack wear, the piston moves axially alongthe output shaft. The sensor detects the axial location of the pistonalong a length of the output shaft by way of a characteristic of asignal that corresponds to, for example, a position of at least one ofthe detectable elements.

According to yet another aspect of the invention, the axial location ofthe piston is one of an operating position, an intermediate position,and a replacement position. When the sensor detects the axial locationin the replacement position, a notification is provided to a user.

According to another aspect of the invention, the power transmissiondevice may also include a piston carrier disposed between and directlycoupled to the piston and the output shaft. In addition, the sensor maybe coupled to a casing of the power transmission device and extendthrough an orifice formed therein.

These and other aspects and objects of the present invention will bebetter appreciated and understood when considered in conjunction withthe following description and the accompanying drawings. It should beunderstood, however, that the following description, while indicatingpreferred embodiments of the present invention, is given by way ofillustration and not of limitation. Many changes and modifications maybe made within the scope of the present invention without departing fromthe spirit thereof, and the invention includes all such modifications.Further, although many methods and materials similar or equivalent tothose described herein may be used in the practice of the presentinvention, a few such suitable methods and materials are described ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A clear conception of the advantages and features constituting thepresent invention, and of the construction and operation of typicalmechanisms provided with the present invention, will become more readilyapparent by referring to the exemplary, and therefore non-limiting,embodiments illustrated in the drawings accompanying and forming a partof this specification, wherein like reference numerals designate thesame elements in the several views, and in which:

FIG. 1 is a perspective view of a power transmission device, accordingto an embodiment of the invention;

FIG. 2 is a side view of the power transmission device of FIG. 1;

FIG. 3 is a cross-sectional view of the power transmission device ofFIG. 2;

FIG. 4 is a perspective view of a piston used within the powertransmission device of FIG. 1, according to an embodiment of theinvention;

FIG. 5 is a perspective view of a piston usable with the powertransmission device of FIG. 1, according to another embodiment of theinvention; and

FIG. 6 is a perspective view of a piston usable with the powertransmission device of FIG. 1, according to yet another embodiment ofthe invention.

In describing the preferred embodiment of the invention which isillustrated in the drawings, specific terminology will be resorted tofor the sake of clarity. However, it is not intended that the inventionbe limited to the specific terms so selected and it is to be understoodthat each specific term includes all technical equivalents which operatein a similar manner to accomplish a similar purpose.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention and the various features and advantageous detailsthereof are explained more fully with reference to the non-limitingembodiments described in detail in the following description.

First referring to FIGS. 1-3, perspective, side, and cross-sectionalviews of a power transmission device 10 with a clutch stack wear sensorsystem are shown. The device 10 may be a PTO (power take off) unit, aPTI (power take in) unit, or other transmission unit. The device 10includes a power transmission shaft 12 oriented along an axis 14 andconfigured to rotate about the axis 14 to transmit torque. The shaft 12may be an output shaft, input shaft, or intermediate shaft 12 dependingon whether the device 10 is a PTO unit, a PTI unit, or a transmissionunit. The shaft 12 extends into a casing 16 which includes additionalelements of the device 10 that can be seen in the cross-sectional viewof FIG. 3. For example, the device 10 may also include a piston carrier18, a clutch piston 20 with a piston body that defines a piston bodyouter surface, a clutch having a clutch stack 22, a hub 24, a hub plate26, and drive ring 28 that is configured to transfer power between anoutside source such as, but not limited to an internal combustionengine, and the shaft 12 of the power transmission device 10. In turn,the clutch stack 22, in conjunction with piston 20, piston carrier 18,and hub 24, may control movement of the shaft 12 in response to movementof the drive ring 28 to engage and disengage the clutch stack 22 andcorrespondingly selectively rotate the shaft 12. That is, when theclutch stack 22 is in an engaged state, power is delivered through theclutch. Meanwhile, when the clutch stack 22 is in a disengaged state,power is not delivered through the clutch. The clutch piston 20 moves toselectively actuate the clutch stack 22 between the engaged state andthe disengaged state.

Starting at a first end 30 of the device 10, a stop plate 32 is coupledto a first end 34 of the shaft 12. The stop plate 32 has a diametergreater than a diameter of the shaft 12, so as to be in contact withboth the first end 30 of the shaft 12 and a first end 36 of hub 24. As aresult, the stop plate 32 provides a backstop upon which the hub 24cannot travel beyond.

The hub plate 26 may then be coupled to the first end 36 of the hub 24by way of a plurality of fasteners 38, such as, but not limited to,restraining bolts. As shown in FIGS. 1 and 3, the hub plate 26 isannular in shape, and the stop plate 32 is aligned in a center orifice40 thereof. The hub plate 26 extends from the hub 24 to the clutch stack22 so as to be in contact with both the first end 36 of the hub 24 and afirst end 42 of the clutch stack 22 at a first end of the clutch stack22 and provide a backstop thereto.

As shown in FIG. 3, the drive ring 28 includes a main portion 44oriented parallel to the shaft 12 and extending into the casing 16. Aflanged portion 46 extends perpendicularly from the main portion 44 at afirst end of the drive ring 28. In turn, the flanged portion 46 of thedrive ring 28 is configured to be coupled to and receive power from thepreviously mentioned prime mover. FIGS. 1 and 3 also show the drive ring28 as annular in shape, and the hub plate 26 as being disposed within acenter orifice 48 of the drive ring 28.

The hub 24 is coupled to the shaft 12 adjacent the first end 34 of theshaft 12. The hub 24 is annular in shape and receives the shaft 12within its center orifice 50. The center orifice 50 is configured tohave a diameter equal to or substantially equal to the diameter of theshaft 12, which may include a tapered fit, splined connection, or keyedconnection. As a result, rotation of the hub 24 is directly translatedto the shaft 12 and vice versa.

The clutch stack 22 includes multiple clutch plates such as multiplefriction and smooth plates that are stacked against each other in analternating pattern. The multiple friction plates are then coupled tothe drive ring 28, while the smooth friction plates are coupled to theother of the drive ring 28 and the hub 24, or vice versa. The frictionplates coupled to the drive ring 28 therefore rotate with the drive ring28 and prime mover, while the friction plates coupled to the hub 24rotate with the hub 24 and shaft 12. When compressed, the frictionplates of the clutch stack are pressed against each other and rotationof the drive ring 28 and prime mover is translated to the hub 24 andshaft 12 by way of the clutch stack 22. Compression of the clutch stack22 for its engagement to rotate the shaft 12 is caused by the piston 20pushing axially away from the piston carrier 18, which may be done byway of hydraulic or pneumatic pressure.

As previously discussed, the first end 36 of the hub 24 is aligned withthe hub plate 26 and the stop plate 32. The piston carrier 18 may belocked into rotational unison with the shaft 12. For example, both thepiston carrier 18 and the shaft 12 may include cavities formed thereinand configured so that a steel ball may occupy both the cavity of thepiston carrier 18 and the cavity of the shift 12. As a result of thesteel ball disposed in both the cavity of the piston carrier 18 and thecavity of the shaft 12, the piston carrier 18 and the shaft 12 arelocked in rotational unison. In yet other embodiments of the invention,the piston carrier 18 may be coupled to the shaft 12. Further, thepiston carrier 18 is disposed at a location adjacent a second end 52 ofthe hub 24. The piston carrier 18 is annular in shape with a centerorifice 54 having a diameter equal to or substantially equal to that ofthe shaft 12. In addition, the piston carrier 18 may be axially fixed inposition along the length of the shaft 12 to provide a structure awayfrom which the piston 20 is biased to engage the clutch stack 22.

The piston 20 is concentrically seated on the piston carrier 18 with aninner circumferential surface of the piston 20 engaging an outer surfaceof an inner collar of the piston carrier and a rearwardly extendingcollar of the piston 20 engaging an outer flange of the piston carrier18 in a manner that allows the piston 20 to axially slide relative tothe piston carrier 18. Piston 20 is aligned with a second end 56 of theclutch stack 22 at a second side of the clutch stack 22. In particular,the piston 20 is configured to rotate and move axially along the lengthof the shaft 12 and the piston carrier 18. When the piston 20 movesaxially toward the clutch stack 22, a first end 58 of the piston 20 isin direct contact with the clutch stack 22 and exerts a force on theclutch stack 22. As such, the clutch stack 22 is compressed by movementof the piston 20 toward the clutch stack 22 and clutch stack 22 engages.

As described above, the rotation of the shaft 12 is translated from therotation of hub through the engages clutch stack 22 when the piston 20is pushed away from the piston carrier 18. As a result, the hub 24 andpiston 20 rotate at the same speed as the shaft 12. FIG. 3 illustrates aclutch stack wear sensor system with a sensor 60 arranged to detectfeatures of the piston 20 while the piston 20 rotates and provide asignal relating to the detectable features. These features will bedescribed in further detail below with respect to detectable features70. Sensor 60 is shown here disposed perpendicular to the piston bodyouter surface, represented here as outer surface 62 of the piston 20 andis adjacent thereto, so that an end 64 of the sensor 60 is spaced apartfrom the outer surface 62 of the piston 20. In other embodiments of theinvention, the sensor 60 may be disposed at angles other thanperpendicular to outer surface 62 of the piston 20.

As shown in FIG. 2, the sensor 60 is secured to and extends through thecasing 16 of the device 10 in order to properly space apart the end 64of the sensor 60 and the outer surface 62 of the piston 20. The sensor60 extends from the first end 64 within the casing 16, through anorifice 66 in the casing 16, and to a second end 68 within the externalenvironment. By extending into the external environment, the sensor 60is able to be manipulated by a user to adjust the spacing between thefirst end 64 of the sensor 60 and the outer surface 62 of the piston 20.In addition, a user may remove and replace the sensor 60 without havingto deconstruct the device 10.

The piston body has various features that can be detected by the sensor60 for determining rotational speed of shaft 12 and the condition orwear state of clutch stack 22. As shown in FIG. 4, the piston 20includes a series of detectable elements 70 equally spaced along theouter surface 62 of the piston 20. In other embodiments of theinvention, the piston 20 may include a single detectable element 70 orany other number of detectable elements 70. The detectable elements 70extend from a first end 72 of the outer surface 62 of the piston 20,which is adjacent the clutch stack 22, to a second end 74 of the outersurface 62 of the piston 20. When the piston 20 rotates, a magneticfield is created due to the material composition of the piston 20. Themagnetic field is changed at each detectable element 70 due to thechange in the contours or material of the outer surface 62 of the piston20, which is made from a ferrous or other material that can be detectedby sensor 60. As described above, the sensor 60 is orientedperpendicular to the outer surface 62 of the piston 20, while alsohaving its first end 64 spaced apart and located adjacent to the outersurface 62 of the piston 20. As such, the sensor 60 is able to detectthe changes in the magnetic field resulting from the detectable elements70. In turn, the rotation speed of the piston 20, piston carrier 18, andshaft 12 can be calculated from the distance between detectable elements70, time between detected change in magnetic field, and thecircumference of the piston 20. In other embodiments of the invention,the sensor 60 may be oriented at an angle other than perpendicular tothe outer surface 62 of the piston 20. In particular, the sensor 60 maybe oriented at any angle allowing the sensor 60 to detect the change inmagnetic field.

At least one of the detectable elements 70 may include a defect 76disposed adjacent the second end 74 of the outer surface 62 of thepiston 20. As the friction plates of the clutch stack 22 wear, thepiston 20 moves further axially along the length of the shaft 12 closerto the first end 34 of the shaft 12 to compress the clutch stack 22.That is, the thickness of the clutch stack 22 is reduced as the frictionplates of the clutch stack 22 wear, resulting in the piston 20 needingto move closer to the first end 34 of the shaft 12 to properly compressthe clutch stack 22.

When the clutch stack 22 includes new friction plates, the sensor 60 isaligned with the outer surface 62 of the piston 20 closer to the firstend 72 of the outer surface 62 of the piston 20 when clutch stack 22 iscompressed. As the friction plates of the clutch stack 22 wear and thepiston 20 moves toward the first end 34 of the shaft 12 when clutchstack 22 is compressed, the sensor 60 maintains its same horizontalposition. As a result, as the friction plates of the clutch stack 22wear, the sensor 60 becomes aligned with the outer surface 62 of thepiston 20 at a location transitioning away from the first end 72 of theouter surface 62 of the piston 20 and transition toward the second end74 of the outer surface 62 of the piston 20. That is, the sensor 60moves relative to the piston 20 along a length of the outer surface 62of the piston 20. For example, FIG. 3 illustrates a wear state of thefriction plates of the clutch stack 22 wherein the sensor 60 is alignedat a location centrally located between the first and second ends 72, 74of the outer surface 62 of the piston 20. A control system with acontroller such as an industrial computer with a controller that mayinclude an industrial computer or, e.g., a programmable logic controller(PLC), along with corresponding software and suitable memory for storingsuch software and hardware including interconnecting conductors forpower and signal transmission between the control system, sensor 60,and/or other components of device 10 is configured to evaluate signalsfrom sensor 60 based on the textured elements to determine therotational speed of shaft 12 and based on defect(s) 76 to determine acondition or wear state of the clutch stack 22.

As the friction plates of the clutch stack 22 become substantially wornand need to be replaced, the sensor 60 becomes aligned with the outersurface 62 of the piston 20 at a location adjacent the second end 74 ofthe outer surface 62 of the piston 20. When the sensor 60 becomesaligned with the defect 76 on the outer surface 62 of the piston 20, achange in the magnetic field associated with the defect 76 will bedetected by the sensor 60. In turn, a notification may be made to notifya user that the friction plates of the clutch stack 22 are substantiallyworn and need to be replaced, for example, through a visual, audible,and/or other notification through a user interface. The control systemrecognizes the signal anomaly or discontinuity created by the defect(s)76, indicating a worn state of clutch stack 22, which differs from theconsistent signal that would otherwise be produced by a constant speedrotation of the shaft 12.

In the representative embodiment of the invention shown in FIG. 4, thedefect 76 may be incorporated into at least one of the detectableelements 70 of the outer surface 62 of the piston 20. However, in otherembodiments of the invention, the defect 76 may be independently formedinto the outer surface 62 of the piston, separate from the detectableelements 70.

Next, FIG. 4 depicts a perspective view of the piston 20 to furtherillustrate the detectable elements 70 and defect 76 described above. Asshown, the detectable elements 70 are evenly spaced around the outersurface 62 of the piston 20. While the representative embodiment of theinvention illustrates the detectable elements 70 as extending from thefirst end 72 of the outer surface 62 to the second end 74 of the outersurface 62, it is contemplated that the detectable elements 70 mayterminate at locations at or adjacent the first and second ends 72, 74of the outer surface 62. In addition, while the detectable elements 70are illustrated as texture elements such as grooves formed into theouter surface 62 of the piston 20, it is also contemplated that thedetectable elements 70 may be texture elements such as ribs or teethraised from the outer surface 62 of the piston 20. In yet otherembodiments of the invention, the detectable elements 70 may be amagnetic or ferrous material different from the piston 20 so as toaffect the magnetic field and be detectable by the sensor 60. In suchinstances, the detectable elements 70 may be disposed within the piston20 to be flush with the outer surface 62 of the piston 20. Similarly,the defect 76 may be a texture element such as that described above or amagnetic or ferrous material flush with the outer surface 62 of thepiston 20.

FIG. 4 further illustrates the defect 76 as being a portion of one ofthe detectable elements 70. For example, the defect 76 may be an angledportion 78 of at least one of the detectable elements 70. The angledportion 78 of the detectable element 70 a is designed to transition thedetectable element 70 a from its initial location 80 to an offsetlocation 82 closer to an adjacent detectable element 70 b. The angledportion 78 begins at the initial location 80 at or adjacent to a center84 of the outer surface 62 of the piston 20, while the ending at theoffset location 82 at or adjacent to the second end 74 of the outersurface 62 of the piston 20.

While FIG. 4 illustrates the angled portion 78 as extending from theinitial location 80 at approximately a 30° angle, it is contemplatedthat the angled portion 78 may extend from the initial location 80 atany angle, including, but not limited to, between 0° and 90°. Forexample, the angled portion 78 may be oriented at a 90° angle. It iscontemplated that the location of the initial location 80 transitionsfrom adjacent the center 84 of the outer surface 62 of the piston 20 toadjacent the second end 74 of the outer surface 62 of the piston 20 asthe angle of the angled portion 78 increases. For example, if the angledportion 78 were oriented at a 90° angle, the initial location 80 wouldbe closer to the second end 74 of the outer surface 62 of the piston 20than if the angled portion 78 were oriented at a 30° angle.

As the piston 20 moves axially toward the first end 34 of the shaft 12as the friction plates wear, the sensor 60 is able to detect and providea signal indicative of the axial location of the piston 20 based on thedefect 76 and, as a result, the wear characteristic of the clutch stack22. When the sensor 60 is aligned at a location between the first end 72of the outer surface 62 and the initial location 80 of the detectableelement 70, the sensor 60 may detect the axial location of the piston asbeing at an operating position 44. The operating position 86 beingdesignated as a position associated with the friction plates of theclutch stack 22 having acceptable wear. In the operating position 86,the sensor 60 does not detect adjustments to the magnetic field otherthan the changes associated with the detectable elements 70 formeasuring rotation speed.

When the sensor 60 is aligned at a location along the angled portion 78between the initial location 80 and the offset location 82, the sensor60 may detect the axial location of the piston 20 as being at anintermediate position 88. The intermediate position 88 being designatedas a position associated with the friction plates of the clutch stack 22having significant wear but still being serviceable. In the intermediateposition 88, the sensor 60 is able to detect its position along theangled portion 78 from the initial location 80 to the offset location 82due to the adjustment to the change of magnetic field associated withthe changing of the distance between the detectable element 70 a and theadjacent detectable element 70 b as detected by the sensor 60. As such,the sensor 60 is able to track the movement of the piston 20 through theintermediate position 88. In addition, the sensor 60 is able todetermine the rate at which it moves from the initial location 80 andthe offset location 82, which relates to the rate at which the frictionplates of the clutch stack 22 wear. In turn, a prediction can be made asto when the friction plates of the clutch stack 22 will wear to thepoint that the sensor 60 is aligned with the offset location 82.

When the sensor 60 is aligned with the offset location 82, the sensor 60may detect the axial location of the piston 20 as being at a replacementposition 90. The replacement position 90 being designated as a positionassociated with the friction plates of the clutch stack 22 needingreplacement due to wear. In the replacement position 90, the sensor 60detects the location of the angled portion 78 at the offset position 82by detecting the adjustment to the magnetic field resulting from thedetectable element 70 a being disposed at the offset position 82.

As described above, the angled portion 78 may be oriented at any anglein varying embodiments of the invention. The change in angle of theangled portion 78 across embodiments of the invention results inadjustments of the intermediate position 88. As stated above, inembodiments of the invention having the angled portion 78 oriented at alarger angle, the initial location 80 is located closer to the secondend 74 of the outer surface 62 of the piston 20. As a result, the axialposition of the piston 20 would be detected as being in the intermediateposition 88 for a shorter period of time. In instances where the angledportion 78 is oriented at a 90° angle, the initial location 80 and theoffset location 82 would be equally adjacent the second end 74 of theouter surface 62 of the piston 20. As a result, there would be nointermediate position 88, as the axial position would transition fromthe operating position 86 to the replacement position 90. In anotheralternative embodiment of the invention, the initial location 80 may beat the first end 72 of the outer surface 62 of the piston 20 and theoffset location 82 may be at the second end 74 of the outer surface 62of the piston 20. As a result, the angled portion 78 would extend fromthe first end 72 to the second end 74 of the outer surface 62 of thepiston 20. The operating position 86 would then be aligned with thefirst end 72 of the outer surface 62 of the piston 20, the replacementposition 90 would be aligned with the second end 74 of the outer surface62 of the piston 20, and the intermediate position 88 would extend fromthe first end 72 to the second end 74 of the outer surface 62 of thepiston 20.

FIGS. 5 and 6 show implementations that may be implemented withoutdefects that cause signal irregularities within an otherwise existingsignal, for example, a rotational speed signal. Instead, thesedetectable elements may be detected without the rotational speed sensor,but instead by some other sensor or mechanism such as a dedicated wearsensor or secondary sensor. Referring specifically to FIG. 5, a piston120 is shown according to an alternative embodiment of the inventionthat implements non-defect detectable elements. The piston 120 includesa first set of detectable elements 170 a, a second set of detectableelements 170 b, and a third set of detectable elements 170 c spacedaround an outer surface 162 of the piston 120. The first, second, andthird sets of detectable elements 170 a, 170 b, 170 c can providedifferent signal characteristics relative to, for example, rotationalspeed that is measured at some other location in the device 10 (FIG. 1)to define the clutch wear state. Similar to detectable elements 70described above, the detectable elements 170 a, 170 b, 170 c may beraised or lowered textured elements or alternative magnetic or ferrousmaterials. As shown in FIG. 5, the first set of detectable elements 170a may be longer than the others, for example, may extend between a firstend 172 and a second end 174 of the outer surface 162 of the piston 120.Detection of only the first set of detectable elements 170 a correspondsto an acceptable wear state of the clutch. The second set of detectableelements 170 b may be shorter than those of the first set 170 a andextend from an initial location or second set outer ends 180 at anintermediate segment of the piston toward the second end 174 of theouter surface 162 of the piston 120. Detection of the first and secondsets of detectable elements 170 a, 170 b provides a signal with a higherfrequency at a given rotational speed than only the first set ofdetectable elements 170 a, for example, double or some other multiple ofthe frequency, which corresponds to a nearing-replacement-neededcondition or service soon indication as the wear state of the clutch.The third set of detectable elements 170 c may be shorter than those ofboth the first and second sets 170 a, 170 b and extend from an offsetlocation or third set outer ends 182 toward the second end 174 of theouter surface 162 of the piston 120. Detection of the first, second, andthird sets of detectable elements 170 a, 170 b, 170 c provides a signalwith an even higher frequency at a given rotational speed than only thefirst set of detectable elements 170 a, for example, quadruple or someother multiple of the frequency, which corresponds to areplacement-needed condition or service now indication as the wear stateof the clutch.

Still referring to FIG. 5, as the piston 120 moves axially toward thefirst end 34 of the shaft 12 the sensor 60 is able to detect the axiallocation of the piston 120 based on which of the first, second, andthird sets of detectable elements 170 a, 170 b, 170 c are detected. Whensensor 60 (FIG. 3) is implemented as a secondary sensor or wear sensorthat that detects the detectable elements 170 a, 170 b, 170 c withoutbeing used in a rotational speed determination, the sensor's 60signal(s) may be compared with a signal(s) from a different rotationalspeed sensor to determine how many detectable elements or which set(s)is being detected at a given rotational speed. This comparison allowsthe system to determine which of the first, second, and third sets ofdetectable elements 170 a, 170 b, 170 c are being detected and thereforethe axial location of the piston 120. In an acceptable wear state of theclutch, the piston 120 fully engages the clutch and is arranged withrespect to the sensor 60 (FIG. 3) so that only the first set ofdetectable elements 170 a are detected at an operating position 186 asan indication that the friction plates of the clutch stack 22 haveacceptable wear. In a service or repair soon state of the clutch, thepiston 120 fully engages the clutch and is arranged with respect to thesensor 60 (FIG. 3) so that both the first and second sets of detectableelements 170 a, 170 b, are detected at an intermediate position 188 asan indication that the friction plates of the clutch stack 22 may havesignificant wear but are still usable. In a service or repairimmediately state of the clutch, the piston 120 fully engages the clutchand is arranged with respect to the sensor 60 (FIG. 3) so that all ofthe first, second, and third sets of detectable elements 170 a, 170 b,170 c are detected at a replacement position 190 as an indication thatthe friction plates of the clutch stack 22 needing replacement due towear.

Next, FIG. 6 depicts another embodiment of the invention having a piston220 with a single detectable element(s) 276 which may be detectable bysensor 60 (FIG. 3) to provide a discrete clutch state determination suchas a go/no-go indication or a service soon indication. Although shown asa single detectable element 276, multiple detectable elements 276 may beprovided to facilitate the same determination, whereby detection ofelement(s) 276 corresponds to the discrete clutch state. In suchembodiments of the invention, the detectable element 276 may be anindependent textured or other detectable element disposed on an outersurface 262 of the piston 220, shown here adjacent a second end 274thereof, which corresponds to a replacement position 290. In such anembodiment of the invention, the detection of the detectable element 276may be used to indicate the immediate change from of an axial position280 from an acceptable state or operating position 286 to aservice-needed state or the replacement position 290 without monitoringthe axial position of the piston through an intermediate position, whichmay be used in a go/no-go determination.

It is specifically intended that the present invention not be limited tothe embodiments and illustrations contained herein, but includesmodified forms of those embodiments including portions of theembodiments and combinations of elements of different embodiments ascome within the scope of the following claims.

What is claimed is:
 1. A method of determining wear of a clutch stack ina power transmission device, the power transmission device having apower transmission shaft that is configured to transmit torque and aclutch that is configured to selectively deliver power into or out ofthe power transmission shaft, wherein the clutch includes a clutchpiston that actuates to engage or disengage a clutch stack toselectively deliver power into or out of the power transmission shaft,and wherein the method comprises: measuring a rotational speed thatcorresponds to a rotational speed of the power transmission shaft;generating a speed signal corresponding to the measured rotationalspeed; detecting at least one detectable element on the clutch piston;generating a wear signal corresponding to the detected at least onedetectable element; evaluating the speed signal and the wear signal;determining a wear state of clutch stack based on the evaluation of thespeed signal and the wear signal; and providing a notification to a userthat corresponds to the determined wear state of the clutch stack. 2.The method of claim 1, further comprising: measuring the rotationalspeed and providing the speed signal with a first sensor; and detectingthe detectable element and providing the wear signal with a secondsensor.
 3. The method of claim 1, wherein the at least one detectableelement is defined by a single set of detectable elements and the methodfurther comprises: detecting the set of detectable elements; determininga no-go state of the power transmission device based on the detection ofthe detectable element corresponding to a wear condition of the clutchstack requiring service of the clutch stack; and providing a servicerequired message to the user as the notification.
 4. The method of claim1, wherein the at least one detectable element is defined by a singledetectable element and the method further comprises: detecting thedetectable element; determining a no-go state of the power transmissiondevice based on the detection of the detectable element corresponding toa wear condition of the clutch stack requiring service of the clutchstack; and providing a service required message to the user as thenotification.
 5. The method of claim 4, further comprising: determiningthe no-go state as a step change in status from an acceptable state ofwear of the clutch stack to a service-required state of the clutch stackbased on detection of the detectable element.
 6. The method of claim 1,wherein the at least one detectable element is defined by multiplesingle set of detectable elements and the method further comprises:detecting at least one of the set of detectable elements; determining adiscrete wear condition of the clutch stack for each of the at least oneset of detectable elements; and providing a wear condition message tothe user as the notification.
 7. The method of claim 6, furthercomprising: detecting a first set of detectable elements of the at leastone set of detectable elements; determining a first wear condition ofthe clutch stack based on the detection of the first set of detectableelements; and providing a first wear condition message indicating thefirst wear condition to the user as the notification.
 8. The method ofclaim 7, further comprising: detecting a second set of detectableelements of the at least one set of detectable elements; determining asecond wear condition of the clutch stack based on the detection of thesecond set of detectable elements; and providing a second wear conditionmessage indicating the second wear condition to the user as thenotification.
 9. The method of claim 8, further comprising: displayingthe first wear condition message until detection of the second set ofdetectable elements.
 10. The method of claim 8, wherein each of thefirst and second sets of detectable elements is defined at an outercircumferential surface of the clutch piston, the method furthercomprising; measuring the rotational speed and not detecting either ofthe first and second sets of detectable elements when the clutch definesan un-wore-state of the clutch stack; measuring the rotational speed anddetecting the first set of detectable elements when the clutch defines apartially wore-state of the clutch stack; and measuring the rotationalspeed and detecting the second set of detectable elements when theclutch defines a full wore-state of the clutch stack.
 11. The method ofclaim 10, further comprising: displaying a service soon message inresponse to detection of at least one of the first and second sets ofdetectable elements.
 12. The method of claim 11, further comprising:displaying a service soon message in response to detection the first setof detectable elements; and displaying a service now message in responseto detection the second set of detectable elements.
 13. The method ofclaim 1, wherein the clutch piston has an outer circumferential surfaceand the at least one detectable element is defined as a set ofdetectable elements on the clutch piston outer circumferential surfaceand the method further comprises: detecting the set of detectableelements on the clutch piston outer circumferential surface to measurethe rotational speed; identifying an anomaly in the speed signal; andgenerating the wear signal as a function of the anomaly identified inthe speed signal.
 14. The method of claim 13, further comprising:identifying different anomalies in the speed signal indicative ofdifferent wear states of the clutch stack.